Bob Horner played college baseball at Arizona State University and was drafted by the Atlanta Braves with the first pick in the 1978 amateur draft. He bypassed the minor leagues and went straight to the Braves. Despite only playing in 89 games, he hit 23 homers and was named National League Rookie of the Year in 1978. Over the next eight seasons, he averaged 24 home runs a year and was named to the All-Star team in 1982. When the Braves started the 1980 season poorly, going 1-9 over their first 10 games, owner Ted Turner got made and singled out Horner for criticism (he was hitting .059 at that point), saying that he was out of shape and asking that he be sent to the minor leagues. Horner refused, saying it was only an early-season slump, and won his point. He ended up hitting 35 homers that year.

He signed a four-year, $6 million contract in 1983 that included a clause that would pay him $100,000 a year if he kept his weight under 215 pounds, something which caused a lot of derision among reporters. In 1986, he hit four home runs in one game in a loss to the Montreal Expos. Following the season, he became a free agent. Finding no acceptable offers in the United States because of collusion, Horner opted to spend the 1987 season playing in Japan instead. He hit .327 with 31 homers for the Yakult Swallows. He returned to the U.S. in 1988, signing with the St. Louis Cardinals. However, he struggled with injuries and only played in 60 games and hit 3 home runs. The Cardinals released him after the season. Bob Horner ended his career never having played a single game in the minor leagues.